This invention relates to a device for measuring an oxygen concentration within an exhaust gas from an internal combustion engine etc., to sense the air/fuel (hereinafter abbreviated as A/F) ratio and in particular to an improved engine A/F ratio sensing device of an oxygen pump type constructed using an ion conducting solid electrolyte.
It is hitherto well known in the art to control e.g. an engine of an automobile to run at a stoichiometric (theoretical) A/F ratio by sensing its combustion state in relation to the stoichiometric A/F ratio according to the variation of an electromotive force produced by the difference of the oxygen partial pressure between the exhaust gas and the atmosphere by means of an oxygen sensor constructed with an ion conducting solid electrolyte such as stabilized zirconia.
When the A/F ratio which is given by the weight ratio of air to fuel is the stoichiometric A/F ratio of 14.7, the above type oxygen sensor can provide a large output variation, while outside the stoichiometric A/F ratio it provides a substantially null output variation. Therefore, when the engine is operated at an A/F ratio outside the stoichiometric A/F ratio, the output of such an oxygen sensor can not be utilized.
Thus, an A/F ratio sensor of an oxygen pump type which eliminates such a disadvantage and enables the engine to be operated at any A/F ratio has already been proposed. However, such a sensor is not practical for the reason of an outstanding variation of its characteristic due to temperature variation.
FIG. 1 shows an arrangement of an A/F ratio sensing device of an oxygen pump type, and FIG. 2 shows a cross sectional view of the sensor in FIG. 1 taken along line II--II, which is disclosed in a related application Ser. No. 606,926 filed May 4, 1984.
In FIG. 1, within an exhaust pipe 1 of an engine (not shown) an A/F ratio sensor, generally designated by a reference numeral 2, is disposed. This sensor 2 is formed of a solid electrolyte oxygen pump cell 3, a solid electrolyte oxygen sensor cell 4, and a supporting base 5. The solid electrolyte oxygen pump cell 3 includes an ion conducting solid electrolyte (stabilized zirconia) 6 in the form of a plate with a thickness of about 0.5 mm having platinum electrodes 7 and 8 disposed on the respective sides thereof. The solid electrolyte oxygen sensor cell 4, similar to the pump cell 3, includes an ion conductive solid electrolyte 9 in the form of a plate having platinum electrodes 10 and 11 disposed on the respective sides thereof. The supporting base 5 supports the oxygen pump cell 3 and the oxygen sensor cell 4 so that they are oppositely disposed having a minute gap "d" of about 0.1 mm therebetween.
An electronic control unit 12 is electrically coupled to the pump cell 3 and the sensor cell 4. More specifically, the electrode 10 is connected through a resistor R1 to the inverting input of an operational amplifier A, the non-inverting input of which is grounded through a DC reference voltage source V. This DC reference voltage serves to control the output voltage of the sensor cell 4 to assume said voltage V according to the oxygen partial pressure difference between those within the gap and outside the gap. The electrode 7 is connected through a resistor Rs to the emitter of a transistor Tr whose collector is grounded through a DC power source B and whose base is connected to the output of the operational amplifier A and the inverting input of the operational amplifier A through a capacitor C. The electrodes 8 and 11 are grounded.
U.S. Pat. No. 4,272,329 discloses the principle of an A/F ratio sensing device of an oxygen type.
In operation, when the oxygen partial pressure within the gap portion between the cells 3 and 4 is the same as the oxygen partial pressure outside the gap portion, the sensor cell 4 generates no electromotive force. Therefore, the inverting input of the operational amplifier A receives no voltage and, accordingly the operational amplifier A provides as an output therefrom a maximum voltage corresponding to the reference voltage V to the base of the transistor Tr. Therefore, the transistor Tr is made conductive to cause a pump current Ip to flow through the electrodes 7 and 8 of the pump cell 3 from the voltage source B. Then the pump cell 3 pumps oxygen present in the gap portion "d" into the exhaust pipe 1. As a result, the sensor cell 4 develops an electromotive force "e" thereacross according to the oxygen partial pressure difference on both sides of the cell 4.
Therefore, the oxygen sensor cell 4 applies the electromotive force "e" generated across the electrodes 10 and 11 to the inverting input of the operational amplifier A through the resistor R1. The operational amplifier A provides an output now proportional to the difference between the electromotive force "e" and the reference DC voltage V applied to the non-inverting input. The output of the operational amplifier A drives the transistor Tr to control the pump current Ip.
Thus, the electromotive force "e" approaches the reference voltage V. Accordingly, the control unit 12 reaches an equilibrium state and serves to provide a pump current Ip necessary for keeping the electromotive force "e" at the predetermined reference voltage V. The resistor Rs serves to provide an output corresponding to the pump current Ip supplied from the DC power source B as a pump current supply means. The pump current Ip corresponds to an A/F ratio value. This pump current Ip is converted into a voltage by the resistor Rs and is sent to a fuel control unit (not shown) so that the fuel control unit may be controlled at a desired A/F ratio. The resistance of the resistor Rs is selected so as to prevent the pump current Ip from flowing excessively from the DC power source B. The capacitor C forms an integrator associated with the operational amplifier A and serves to make the electromotive force "e" precisely coincident with the reference voltage V.
One example of the static characteristics of an A/F ratio sensing device of an oxygen pump type thus constructed in the form of a negative feedback control is shown in FIG. 3. A solid line indicates a characteristic of pump current Ip as a function of A/F ratio when the A/F ratio sensor 2 is exposed at 600.degree. C. while a dotted line indicates a characteristic at 800.degree. C. It is found that such a characteristic variation gives rise to the thermal variation of the ion conductivity of the ion conducting solid electrolytes 6 and 9, respectively forming the solid electrolyte oxygen pump cell 3 and the solid electrolyte oxygen sensor cell 4. Experiments reveal that as the temperature of the A/F ratio sensor is varied over a temperature range of an engine exhaust gas, the current Ip, flowing through the oxygen pump cell 3, corresponding to the same A/F ratio varies up to several ten percentages thereof, resulting in an unpractical A/F ratio sensor.
On the other hand, FIG. 4 shows a temperature dependency of the electrical resistance of the ion conducting solid electrolytes 6 and 9. Since this characteristic is common to various solid electrolytes, the application of this characteristic shown in FIG. 4 enables the temperature variation of the A/F ratio sensor to be corrected.